Bleaching silver images in the formation of printing plates



United States Patent 3,083,097 BLEACHENG SHEER IMAGES IN THE FGRMA- TEON OF PRINTING PLATES Woifgang Ltissig and Eberhard Gunther, Leveritusen,

Germany, assignors to Agfa Aktiengeseilschaft, Leverlrusen, Germany, a corporation of Germany No Drawing. Filed Apr. 23, 1958, Ser- No. 730,259 Claims priority, application fierrnany Apr. 26, 1957 5 Claims. (Ci. 9629) The present invention relates to planographic printing and more especially to planographic forms which are produced by transforming photographic silver images into hydrophobic ink-receptive images.

It is known that silver images produced photographically can be converted by means of a bleaching or toning process or by reducers or intensifiers with the aid of oxidation baths into sparingly soluble heavy metal and/ or silver salt images. In this process, potassium ferricyanide is used for example for oxidizing the silver, whereby the silver image is converted into ferrocyanide, which is precipitated partly as insoluble silver ferrocyanide and partly as insoluble ferrocyanide of other metals, which are added to the baths in the form of salts. In the known blue toning processes, ferricyanides and ferric salts are used. It is also known to intensify with potassium ferricyanide and lead salts. These processes have however so far only been used for producing certain color effects or intensifying effects with silver salts. These converted silver images have not been used for the printing process using fatty printing inks. Furthermore, it is known to tan the gelatine and the image areas through a silver image, but in this case the silver image itself does not take up the fatty printing ink but the silver image causes the tanning of the gelatine, so that the latter is capable of absorbing fatty printing ink.

Finally it is known to expose dichromate layers with the addition of ferrous salts, oxidation of the ferrous salt taking place to form an image. Apart from the relatively low sensitivity, the use of dichromate layers has the disadvantage that the dichromate layers can only be kept for a short time and that very strong light sources are necessary for the exposure.

It has now been found that planographic printing forms can be made by way of a silver image, the converted silver image being capable of taking up greasy printing ink, if silver images are converted by oxidation into images of heavy metal and/ or silver compounds and these are reacted with organic compounds which contain SH, SeI-I, OH or NH groups and therefore can form organic salt-like or complex compounds with the heavy metal and/ or silver ions of the compounds formed by the oxidation, the organic heavy metal and/or silver compounds being more sparingly soluble than these lastmentioned compounds. Suitable organic compounds are those which are sparingly soluble in water, that is to say the solubility of which in water at a temperature of 20 C is lower than 0.1 percent by weight.

The insolubility of these compounds is effected by suitable substituents such as aryl, aralkyl, cycloalkyl, and alkyl radicals, preferably one or more long-chained straight or branched chain, saturated or unsaturated hydrocarbon radicals with at least 10, preferably to carbon atoms. For carrying out the process of the present 3,083,097v Patented Mar. 26, 1963 invention these compounds are applied in form of their salts with alkali metals (sodium, potassium), ammonia or amines because they are by far better soluble in aqueous solutions in salt form and can be supplied in higher concentrations. For this reason such compounds are especially suitable which contain SH or SeH groups, which can be in a tautomeric form, if the carbon atom carrying these groups is further bonded to two further hetero atoms, heterocyclic compounds having an OH group bonded to a heterocyclic carbon atom and heterocyclic compounds having a cyclically bonded NH group, because these constitutions ensure at the same time a good solubility of the compounds in alkaline solutions and a low solubility of the heavy metal and/or silver salts obtained with these compounds.

Suitable compounds are characterized by the following general formulae:

furyl, thia'zolyl), which substituents are either linked to one of the two atoms Y and Z, directly or by means of other linking groups, or linked to one of the ring elements of the heterocyclic ring.

H-N" R in which:

H HN-7=N represents part of a S-membered heterocyclic ring, such as for example 1,3,4-triazole, imidazole or tetrazole, and R represents aryl (phenyl, tolyl), aralkyl (benzyl), and alkyl, preferably a long-chained straight or branched chain, saturated or unsaturated hydrocarbon radical with at least 8 carbon atoms, which is linked to one of the ring atoms, directly or by way of one or more intermediate members.

in which:

N it stands for a live or six-membered heterocyclic ring such as a triazole, imidazole, and pyrimidine ring; and R represents an aryl (phenyl, tolyl), aralkyl (benzyl), an alkyl (4) Mercaptans (C H SH, C H SH) (5) Thioarnides (-C H CSNH C H CS.NH

By way of explanation, some of the suitable compounds are set out as examples in the following synopsis.

R aryl, aralkyl, or. alkyl, preferably with 10 to 20 carbon atoms, such as (XIII) 1 102 (XIV) 5-heptadecyl-7-hydroxy-2,3,4-triazaindolizine:

(XV) 1-amino-2-octylmercapto-S-hydroxy-I,3,4-triazole:

N N HO-A -S-CsHn N 1\|IH2 (XVI) -1,2,2,3- 5-phenyl-1,3,4-thiodiaziuo) -5-hydroxy- 1,3,4-triazole oan N-o (XVIII) Thioureas:

(XIX) Thiosemicarbazones (XX) Salts of N,N-dialkyl dithiocarbamic acid Particularly suitable for carrying out the process of the invention are silver images which are on the surface of the actual supports, these images preferably being produced by the silver salt diffusion process.

According to this process an imagewise exposed silver halide layer is immersed in a silver halide developing solution containing a silver halide solvent such as sodium thiosulfate and then brought into contact with a light insensitive hydrophilic transfer layer, such as a gelatine layer. During this process unexposed silver halide of the silver halide layer is dissolved and transferred into the transfer layer Where it is reduced by the silver halide developing solution to a silver image. The silver halide solvent may also be incorporated into the transfer layer, whereasthe developing substance, such as hydroquinone, may be incorporated into the silver halide layer. The transfer layer may also contain small catalytic amounts of developing nuclei, such as silver sulphide. By using this process, it is possible for the silver image to be applied to the hydrophilic surfaces of a wide range of different supports and it is thus possible to obtain printing 5 forms for planographic printing after conversion of the silver image into oil-absorbing compounds.

Paper is an example of a suitable support for the planegraphic printing form, if this paper is coated with a waterresisting but nevertheless hydrophilic layer, for example with a hardened gelatine layer or superficially saponified cellulose acetate. Into such layer there may be incorporated matting agents such as TiO SiO BaSO which are inert with respect to ferrocyanides and ferricyanides. These matting agents may be applied in quantities of about 5-100 percent as calculated on the Weight of binding agent.

As the support for the planographic printing form, it is furthermore possible to use plates consisting of metals alloys, or their oxides, if such oxidizing baths are used which are chemically inert with respect to said plates. Using such plates, it is possible even without any preliminary treatment to produce a silver image thereon, when they are brought into contact with an exposed negative layer in a suitable developer which contains a solvent for silver halide. Other supports which are suitable are plastic supports having a sufficiently hydrophilic surface, for example superficially saponified acetyl cellulose foil. It is advantageous to ensure that such plates and foils are provided with development nuclei for example silver sul phide before they are processed, so that the reduction of the silver salts takes place at sufiicient speed during the silver salt diifusion process.

Suitable for oxidizing the silver images are all combinations of an oxidizing agent with an anion by means of which there are formed heavy metal salts and/or silver salts which are still soluble and available for subsequent conversion into the organo-metallic compounds. These are preferably baths by means of which silver chloride, silver bromide, silver ferrocyanide or combined silver and heavy metal ferrocyanides and ferricyanides are formed i.e. combinations of copper sulphate with (31- ions or K Fe(CN) As oxidizing baths there are also suitable aqueous solutions of oxidizing agents which transform the silver of the images at least superficially into silver oxide, such as solutions of Water soluble permanganates, dichromates and iodic acid.

Furthermore, the silver image may be oxidized with a solution of a ferricyanide which is soluble in aqueous solution, the oxidation being effected in the presence of metal salts the metal ions of which form. ferrocyanides and/ or ferricyanides which are insoluble in aqueous solution, so that insoluble ferrocyanides, or co-precipitates of insoluble ferrocyanide and ferricyanide oxidizing salts are precipitated at the place of the silver image. In certain cases it is advantageous to add to the treating bath complex formers Which are capable of dissolving Waterinsoluble metal ferricyanides.

The composition of the conversion baths for the silver image depends mainly on the cation which, in addition to the silver of the silver image, should form an insoluble ferrocyanide with the reduced ferricyanide.

The metal cations concerned can be subdivided into two groups, depending on their behaviour:

Included in group A are the cations which form an insoluble deposit only with a ferrocyanide, such as for example lanthanum lead thorium and uranyl (U For this group, it is necessary to have a conversion bath containing the following constituents:

(1) A salt of a metal of this group, the said salt being soluble in water or in weak acids, such as lanthanum, ace tate, lead nitrate, thorium nitrate, uranylnitrate;

(2) A water-soluble fern'cyanide, for example alkali metal or alkaline earth metal ferricyanides such as K Fe(CN) Na Fe(CN),,-, Li Fe(CN) if desired, it is also possible for an acid to be added.

Included in group B are the cations which would also form an insoluble deposit with the 'ferricyanide of the bath, such as for example silver, copper, zinc, cadmium,

6 manganese iron cobalt and nickel For this group, the conversion bath must contain the following constitu-V ents: i t

(1) A salt of a metal of this gnoup, the said salt being soluble in water or in weak acids, such as silver nitrate, copper sulfate, zinc sulfate, zinc chloride, cadmium sulfate, manganese sulfate, manganese chloride, iron chloride, cobalt nitrate, cobalt sulfate, nickel nitrate, and zinc acetate;

(2) A complex former or chelating agent which prevents the metal ferricyanide from precipitating or forming a colloidal solution, such as for example iron without however preventing the formation of insoluble ferrocyanides during the silver conversion.

(3) A water-soluble ferricyanide, for example alkali, metal or alkaline earth metal fenicyanides such as An acid can, if desired, also be added in order to adjust the pH value to about 3-6. Depending on the nature of the cation, soluble citrates, ttartrates or oxalates or ammonia (for silver), are suitable as complex formers. Suitable acids are inorganic and organic acids, however, such acids are preferred which have the same anion as the complex former.

Among the metal salts mentioned above, those of iron, cobalt, nickel, copper, zinc and the uranyl group yield particularly good results, but the salts of silver, lanthanum, thorium, lead and manganese are also suitable for the process. These metal salts can be used individually or in admixture with one another. Both as regards group A and group B, the part of the bath containing the soluble ferricyanide is preferably kept separate from the solution with the remaining constituents, since prepared baths containing all the constituents are in many cases not stable for a very long time, this being in contrast to the separate component solutions.

The proportion of metal salt is approximately 1 to 10 g. per 200 cc. of the total bath; the proportion of the soluble fern'cyanide is about 0.5 to 2 0 g. per 200 cc. of the total bath. The proportion of the complex former required with cations of group B is different in each case and must be chosen to be so small that, on the basis of the decomposition constants of the metal complex, the reaction comprizing the conversion of silver and the separation of insoluble ferrocyanide (possibly with the entrainment of ferricyanide) takes place quickly. This proportion must, however, not be too small, since otherwise the solution decomposes too rapidly, in which case the metals separate out in the form of insoluble compounds before they are able to participate in the conversion of the silver image. if a rapid conversion is required, the most favourable proportion of complex formers is substantially that with which the previously clear solution of the total bath starts to become cloudy in air after about 25 minutes. At this time, ferrocytanides separate out and these frequently also entrain ferricyanides. In case of citrates these compounds may be used in solutions containing about 1-30 g. per 200 cc. as calculated on the total bath. The conversion times in such baths are 1-5 minutes, while they are correspondingly longer with those baths which contain somewhat more complex former. The end of the conversion can in every case be recognized by the complete disappearance of the color of the superficial and usually black image, and this can be observed particularly easily when the precipitated ferrocyanide and the ferricyanide which may be entrained therewith have a light color. For example with nickel, the ferrocyanide is light green and the ferricyanide is yellow.

The conversion of these deposits into the organo-metallic compounds takes place, as indicated above, owing to the difference of the solubility products, as ion exchange reactions. Therefore, the organic compounds are applied in the form of at least partially aqueous solution, either by immersion or rubbing. As pointed out above in most cases an alkaline agent is added to produce better solubility of the organic compounds, whereby simultaneously an increase in the reaction velocity is produced. By the addition of suitable silver salt solvents, for example ammonia or potassium-thiocyanate, the deeper portion of the image can also be included in the reaction. The organic compounds are preferably used in concentrations of 2-100 g. per litre of solution. As solvent there is preferably used water in combination with alkali agents such as hydroxides of alkaline metals (sodium, potassium, lithium), ammonia, and/ or quaternary ammonium bases such as tetramethylamrnonium hydroxide. In case of very sparingly soluble compounds it is advisable to add watermiscible organic solvents preferably propanol in quantities up to about 60 percent calculated on the volume of the total solution. The pH value of the solution may vary within wide limits, pH values of about 8 to 14 being pre ferred. These solutions may be applied to the converted silver images at room temperature.

As pointed out above the inorganic compounds into which the silver image is transformed in the first step have a higher solubility product than the organic compounds which are obtained in the second step, the solubility product of the inorganic compound preferably amounting to about 10* to and that of the organic compounds to about 10" to 10*.

Example 1 A lacquered water-resisting paper base is coated with a layer containing gelatine as binder, catalytic amounts of colloidal silver and a solvent for silver halide, preferably sodiumthiosulfate (1 g. per 10 g. of gelatine). The layer is hardened with formalin.

A positive of a line original or type-written original is produced on the foil by the silver salt diffusion process, for example as described in German Patent No. 887,733, according to which a silver halide emulsion layer is exposed to the object to be reproduced, immersed in an ordinary silver halide developing solution containing hydroquinone as a developing substance and thereafter brought into contact with said foil. During this process silver halide is dissolved out of the unexposed areas of the silver halide layer and transferred to the foil, where it is reduced by the developing solutions to a silver image which is positive as compared with the object to be reproduced. (A suitable developing solution has for instance the following composition: water 1000 cc., p-methylaminophenolsulfate 2.5 g., hydroquinone 15., KBr 2 g., Na SO 100 g, NaOH 30 g.)

After thorough rinsing of the paper, the silver image is bleached to silver chloride by immersing it into or rubbing it with a solution of 70 g. of K Fe(CN) 1000 ml. of water.

Thereafter, the foil is rinsed for removing the bleaching solution and rubbed with a solution of 3.5 g. of 1amino-2mercapto-5-heptadecyl-1,3,41triazole ml. of 2 N sodium hydroxide solution,

10ml. of propanol and 10 ml. of water,

A positive silver image bleached and rinsed as in Example 1 is rubbed with a solution of 4 g. of Z-mercapto-S-heptadecyl-1,3,4-triazole (II) in 10 ml. of 2 N sodium hydroxide solution, 7

20 ml. of propanol and 10 ml. of Water,

which was diluted before use with water in the ratio of 1:4. The further procedure is as in Example 1. Up to 400 copies are obtained.

Example 3 A silver image bleached and rinsed as in Example 1 is rubbed with a solution of 4 g. of Z-mercapto 5 stearylmercapto 1,3,4 thiodiazole (III) in 10 ml. of 2 N sodium hydroxide solution,

20 ml. of propanol and 10 ml. of water this solution having been diluted before use with water in the ratio of 1:5. The further procedure is as in Example 1. Up to 500 copies can be obtained.

Example 4 A silver image bleached and rinsed as in Example 1 is rubbed with a solution of 5 g. of heptadecyl tetrazole (XIII), in 10 ml. of N sodium hydroxide solution, 20 ml. of propanol and 5 ml. of water,

the solution having been diluted before use with water in the ratio of 1:4. The further procedure is as in Example 1. Up to 300 copies are obtained.

Example 5 and thereafter rinsed.

The further processing is as indicated in Example 1. Up to 500 prints are obtained Example 6 Instead of being bleached with the K Fe(CN) bleaching bath, a silver image prepared as in Example 1 is rubbed with a solution of 100 g. CuSO 100 g. NaCl 7 r 25 ml. H (concentrated 1000 ml. H 0

and the image silver is bleached to silver chloride. The further processing is as indicated in Example 1. Up to 500 copies are obtained.

Example 7 Instead of being bleached with a K Fe(CN) bleaching bath, a foil prepared as in Example 1 is rubbed with a mixture of the two following solutions (A) 4 g. of ZnSOUH O,

7.5 g. of sodium citrate and 1 g. of citric acid to cc. of water. (B) 10 g. of potassium ferricyanide to 100 cc. of water The resulting precipitate of silver-acid-zinc ferrocyanide and zlncferricyanide is rubbed with a solution of 3 g. of 1-amino-2-mercapto-S-heptadecyl-1,3,4-triaxole (I) 10 ml. of'N sodium hydroxide solution,

30 ml. of methanol and 10 ml. of water,

and, after being quickly rinsed, it is inked on the printing machine. Up to 600 copies are obtained.

Example 8 The foil bleached and rinsed as 'in Example 1 is rubbed witha solution of 3 g. of dithiocarbazinic acid stearyl ester (VII) in 5 ml. of N sodium hydroxide solution,

20 ml. of propanol and ml. of water.

The further processing is as in Example 1. Up to 500 copies are obtained.

Example 9 The foil bleached and rinsed as in Example 1 is rubbed with a solution of 3 g. of Z-heptadecyl-1,3,4-triazole (XII) in 5 ml. of N sodium hydroxide solution, ml. of propanol and 10 ml. of Water.

The further processing is as in Example 9. Up to 500 copies are obtained.

Example 10 A silver image bleached and rinsed obtained as in Example 1 is bathed for 30 seconds in the following solution:

5 g. of 5-heptadecyl-7-hydroxy-2,3,4-triazaindolizine 100 cc. of water,

140 cc. of l N NaOH,

125 cc. of propanol.

The further procedure is as in Example 1.

It is also possible to apply the hydroxy compounds in combination with the oxidizing agents.

The silver image may be treated for 30 seconds in the following solution:

3.8 g. of 5-heptadecyl-7-hydrox -2,3,4-triazaindolizine 4 g. of potassiumferricyanide 200 cc. of H 0 30 cc. of 1 N NaOH 70 cc. of n-propanol.

The further procedure is as in Example 1.

Example 11 A silver image obtained as in Example 1 is treated for 30 seconds in aqueous 5% potassium bichromate solution. After rinsing the silver image is bathed for 20 seconds in a solution of 20 g. of potassium methyl xanthogenate, 20 cc. of 2 N NaOH, and

Water to make 1000 cc.

or in a solution of 5 g. of octadecylmercaptan,

20 cc. of 2 N NaOH,

50 cc. of propanol,

which is diluted with water in the proportion 1:10.

Example 12 A foil of cellulose acetate is coated with a thin layer of gelatine and ther after treated with a solution of colloidal silver to provide the surface of said layer with colloidal silver. On the thin treated foil there is produced a silver image as disclosed in Example 1.

A silver image is oxidized by treating the foil for 20 seconds in an aqueous 2% potassium permanganate solution and thereafter in a solution of 10 which is diluted before use with 10 times its volume of water.

(b) 10 g. of Z-rnercapto-S-octadecyl-mercapto-1,3,4-thiodiazole cc. of 2 N NaOH 300 cc. of pr-opanol, cc. of water, which is diluted as above.

Instead of the cited silver image there may also be used one which is produced in a hardened silver halide emulsion layer (2 parts of silver halide per 1 part of gelatine) coated on a foil of cellulose acetate or polyester of ethylene-glycol and terephthalic acid. A half-tone negative image is printed into said layer, which is developed and fixed in the ordinary processing bath. The image obtained is rinsed and thereafter treated as above.

The following procedures can be used for the production of certain of the organic substances in the foregoing synopsis.

(I) 1-amino-2-mercapto-S-heptadecyl-l,3,4-triazole: A suspension of 2.1 g. of triocarbohydrazide in 20 ml. of absolute ethanol has added thereto 7 g. of stearic acid iminoethyl ether hydrochloride, and the mixture is heated under reflux for 15 minutes. On cooling, a mixture of substance I and ammonium chloride is precipitated. It is filtered with suction and washed with water, and the residual precipitate is recrystallized from glacial acetic acid. Colorless needles with a melting point of 126127 C. (yield: 55%) are obtained.

.(II) Prepared according to German patent applications B 24741/ 12p (issued as German Patent 958,650, published February 21, 1957) and B 27085 IVc/ 12p (issued as German Patent 953,607, published December 6, 1956).

(III) Preparation of 2-stearylam-ino-5-mercapto 1,3,4- thiodiazole Hsl s JNHC 0 01711 13.3 g. (0.1 mol) of Z-amino-S-mercapto-l,3,4-thiodiazole (prepared according to French [Patent 1,064,234) are dissolved in cc. of dried pyridine. 33.2 g. (0.11 mol) of stearyl chloride are added to the solution with stirring. The reaction mixture is left standing in a closed vessel for 23 days with shaking from time to time. After reaction is complete, the reaction mixture is poured into a mixture of about 600 g. of ice and cc. of concentrated hydrochloric acid. The precipitated Z-stearylamino-5-mercapto1,3,4-thiodiazole is filtered off with suction, washed until free from acid and recrystallized twice from alcohol. Melting point 136 0.; yield 21.4 g.=73.4% of the theoretical. Calculated 10.72% S, found 10.56% S.

(IV) Prepared according to French patent specification 824,821.

(V) Dithiocarbazinic acid stearyl ester: 25 g. of the ammonium salt of dithiocarbamic acid are dissolved in 100 ml. of 2 N sodium hydroxide solution. After the addition of 67 g. of octadecyl bromide in 400 ml. of alcohol, the mixture is heated for 1 hour under reflux and thereafter filtered with suction when cold. The precipitate is melted under glacial acetic acid and freed from unreac-ted initial material by shaking. A compact waxlike mass with a melting point of 60-80 C. is obtained.

(VI) N [N octadecyl-semicarbazide] -dithiocarboxylic acid-methyl-ester: 2.4 g. of dithiocarbacinic acid methyl ester are dissolved in 10 ml. of dioxane and the solution is incorporated by stirring into a solution of 6 g. of stearyl isocyanate in 20 ml. of dioxane. The mixture heats up spontaneously, and is boiled for a short time. On cooling, the substance VI crystallizes out. After recrystallization of the product from alcohol, colorless crystal clusters are obtained melting at 115117 C.

(VIII) Prepared according to Turpin, Ber. der Chem. Gesellsch. 21, 2490.

(IX) Prepared according to Stephen, Journal of the Chem. Soc. (London) 127, II, 1874.

(X) Prepared according to Ber. der Deutschen Chem. Ges. 26, page 34.

(XI) Prepared according to Ber. der Deutschen Chem. Ges. 27, pages 990-94.- (XIV) heptadecyl-7-hydroxy-2,3,4-triazaindolizine: 8.4 g. of 2-amino-1,3,4-triazole are dissolved in'40 ml. of glacial acetic acid and refluxed for three hours after addition of 35.4 g. of stearoyl acetic acid ethyl ester. After cooling the reaction product precipitates. After recrystallizing from glacial acetic acid the product has the melting point 104-l05 C. Yield: 60%.

(XV) (a) Preparation of l-amino 2 mercapto-4-hydroxy-1,3,4-triazole:

9 g. of carbohydrazide are suspended in 13.6 g. of ammonia (25%) 16 ml. of water and 20 ml. of dioxane.

7.6 g. of CS in 50 ml. of dioxane are added with stirring and gently heating. After completion of the reaction the mixture is heated to 60 C. for minutes. Upon cooling colorless needles, which melt at 128l29 C. with decomposition after recrystallizing from water, precipitate. Yield: 90%. 8.1 g. of the compound obtained are dissolved in 50 ml. of water, mixed with 4.4 g. of caustic soda and the solution obtained is refluxed on the water-bath for 16 hours. After cooling the solution is acidified with hydrochloric acid. The precipitate obtained is filtered off and recrystallized from ammonia (25%) MP. 183 C. with decomposition; yield: 60%.

XV) (b) Alkylation. e.g. with octyl bromide to form 1-amino-2-octylmercapto-S-hydroxy-1,3,4-triazole: 1.3 g. of 1-amino-2-mercapto-S-hydroxy-1,3,4-triazole are dissolved in the heat in 10 ml. of alcohol and 3 ml. of Water, mixed with 2.1 g. of octyl bromide and the mixture is refluxed for 20 minutes. Thereupon the mixture is allowed to cool and the precipitated crystals are filtered ofi with suction. After recrystallizing from methanol, platelets of the M.P. 126-128 C. are obtained. Yield 95%.

(XVI) 1,2-2,3 (5 phenyl 1,3,4 thiodiazino) 5- hydroxy-1,3,4-triazole: 1.5 g. of 1-an1ino-2-merc'apto-5- hydroxy1,3,4-triazole in 10 ml. of alcohol (60%) are mixed with 2.5 g. of bromoacetophenone and the mixture is refluxed for 20 minutes. The colorless needles precipitating upon cooling are filtered off with suction. After recrystallizing once from alcohol they melt at 182-187 C. Yield: 90%.

We claim:

1. A process for the production of planographic printing forms for printing with fatty inks comprising bleaching with a silver oxidizing bath an image of silver particles dispersed in a supported hydrophilic layer, to obtain a corresponding inorganic silver image having a solubility product within the range of about 10* to 10*, said silver oxidizing bath containing an inorganic oxidizing salt, then reacting said inorganic silver image with an alkaline solution of an organic compound selected from the group consisting of A H-N' R il+ no-o (v) ns-n and v1) RCS-NHa N represents a S-mcmbered heterocyclic ring and the group H g stands for a member selected from the class consisting of S-membered heterocyclic rings, 6-membered heterocylic rings and 5- and 6-membered heterocyclic rings fused together, obtaining a corresponding organic silver compound more sparingly soluble than said inorganic silver compound, and thereafter applying a fatty printing ink to said corresponding organic silver compound.

2. A process according to claim '1 wherein said oxidizing bath contains as said inorganic oxidizing salt a substantial amount of non-silver heavy metal ion, for converting said image into a corresponding inorganic silver compound and a compound of said heavy metal.

3. A process according to claim 1, wherein said oxidizing bath contains said inorganic salt an oxidizer and said organic compound.

4. A process according to claim 1, wherein said oxidiz ing bath contains as oxidizing agent a compound selected from the group consisting of water soluble ferricyanides,permanganates, and cuprihalogenides.

5. A process according to claim 1, wherein said silver image is produced by a silver salt ditfusion process.

References Cited in the file of this patent UNITED STATES PATENTS 2,353,754 Peterson July 18, 1944 2,487,446 Kellog Nov. 8, 1949 2,487,569 Mackey Nov. 8, 1949 2,710,256 Eckler et a1. June 7, 1955 2,794,388 Lake et a1. June 4, 1957 FOREIGN PATENTS 556,753 Belgium Oct. 17, 1959 887,733 Germany Aug. 27, 1953 758,959 Great Britain Oct. 10, 1956 OTHER REFERENCES Belgium, 556, 753, abstracted in Photographic Abstracts, publ. by The Royal Photographic Society of Great Britain, London, vol. 38, part 4, 1958, page 325. Tory: Photolithography, publ. by Associated General Publications, Sidney, Australia, published 1953, pages 19-22. 

1. A PROCESS FOR THE PRODUCTION OF PLANOGRAPHIC PRINTING FORMS FOR PRINTING WITH FATTY INKS COMPRISING BLEACHING WITH A SILVER OXIDIZING BATH AN IMAGE OF SILVER PARTICLES DISPERSED IN A SUPPORTED HYDROPHILIC LAYER, TO OBTAIN A CORRESPONDING INORGANIC SILVER IMAGE HAVING A SOLUBILITY PRODUCT WITHIN THE RENGE OF ABOUT 10-5 TO 10-14, SAID SILVER OXIDIZING BATH CONTAINING AN INORGANIC OXIDIZING SALD, THEN REACTING SAID INORRGANIC SILVER IMAGE WITH AN ALKALINE SOLUTION OF AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF 